The present invention relates to a battery charger for rapidly charging a battery and, more particularly, to a computerized battery charger which controls the amount of charging current supplied to the battery and reduces the charging current to a trickle current when the battery is fully charged.
Although there are several methods available for rapidly charging batteries, these methods fail to quickly stop the charging current being supplied to the battery once the battery is fully charged. If a battery continues to be charged after it has been fully charged, the overcharging may destroy or damage the battery. Thus, it is desirable to have a battery charger which is capable of rapidly supplying a charging current to a battery which quickly discontinues the charging current once the battery is fully charged.
Previously, conventional types of battery chargers have attempted to control the charging of a battery by detecting a peak battery voltage condition, and discontinuing the charging current upon the detection of such a peak battery voltage. When a microprocessor has been used in such a battery charger, the battery voltage is usually monitored by the microprocessor which calculates the slope of the battery voltage. As long as the voltage of the battery is increasing, the calculated slope is positive, and upon the decrease of the battery voltage, the calculated slope is negative. Upon calculating a negative slope, the microprocessor assumes that the battery has become fully charged and stops the charging current. The problem with such a method of detection is that the microprocessor must be able to quickly and continuously sample the battery voltage in order to make a slope calculation. When a battery voltage is about to reach its peak, the voltage increases rapidly. The opposite is true once the peak has been reached. Therefore, in order for the microprocessor to calculate the slope just before and just prior to the peak battery voltage, the battery voltage should be sampled quickly.
Additionally, although the use of microprocessors in battery chargers has eliminated some of the conventional problems regarding battery charging, the use of microprocessors has created new problems. Primarily, when microprocessors have been used in battery chargers, the circuits used to incorporate the microprocessor into the battery charger have been complex. The use of complex circuits serves to increase the cost of manufacturing a battery charger, as well as decreasing the reliability of the charger. Furthermore, when microprocessors have been used with battery chargers, time has been wasted by continuously converting and processing data for the microprocessor. One previous battery charger uses a microprocessor within a complex circuit containing a voltage controlled oscillator (VCO). Digital data defining a threshold value is introduced by the microprocessor and subsequently stored. The battery voltage during the charging is converted into digital data and is also stored in memory. When the battery voltage sampled is greater than the last sample taken, the stored value is updated in memory. If the most recently sampled battery voltage is less than the stored data, the difference is compared with the stored digital threshold value. If the difference is greater than the stored digital threshold value, it is assumed that the battery has reached its peak voltage and the charging current is discontinued.
The problems with such a method of discontinuing a charging current are inherent. Because every sample taken must be digitized and stored, the processing time required for each sample taken is relatively high. Generally, the battery voltage is checked approximately once every second. As stated previously, as the peak voltage in a battery is approached, the voltage increases rapidly. Thus, it is imperative to take samples quickly to be able to detect changes in the battery voltage. Because of the processing time, required samples may be taken only once every second, resulting in a substantial delay before the peak battery voltage is actually detected by the microprocessor. A subsequent delay occurs before the voltage to the battery is discontinued because the microprocessor must still digitize and further process the received voltage data.
The concept of using a microprocessor to detect the peak voltage is foremost a safety feature. The goal of such devices is to prevent possible damage to the battery from overcharging. Nevertheless, if the battery overheats, the battery may be damaged. Conventional battery chargers incorporating microprocessors have failed to take this contingency into account.
A need has developed for a battery charger which is able to rapidly charge a battery and discontinue a charging current to the battery very soon after the battery has attained full charge. Such a battery charger is particularly desirable when it incorporates additional safety features such as checking the temperature of the battery during charging and discontinuing charging when the battery is fully charged or if the battery becomes overheated.